1. Field of the Invention
The present invention is related to the production of a laser pulse train. More specifically, the present invention provides a device that generates a plurality of laser pulses from a single laser pulse using an array of optical elements.
2. Description of Prior Art
For many applications, a high-yield electron source with excellent beam quality is needed. For example, short pulse, nearly mono-energetic, tunable x-rays may be generated by the collision of a plurality of electron bunches from a photoinjector with synchronized photon pulses in a high-power laser beam via Compton backscattering. Such x-rays have uses in medical imaging, crystallography, and designer drugs, to name a few. In addition to x-ray generation via Compton scattering, the laser pulses may be used directly or in another manner as described above in other applications such as in optical communications. Another use of a laser-triggered photoinjector is in the generation of a train of multi-bunch electrons for acceleration in linear electron-positron colliders such as the International Linear Collider or the CERN Linear Collider.
Conventional multi-cell photoinjectors are capable of accelerating short electron bunches to energies in the range of 3-30 MeV utilizing the rf pulse from a high power source with peak power typically tens of megawatts. The electron bunches are generated by laser pulses synchronous with the rf pulses. The synchronization is normally controlled by a master clock which drives both the high power rf source and the laser. Thus during each rf pulse, one electron bunch is generated and accelerated in the photoinjector. Because of the great disparity between the length of the rf pulse and that of the laser (and consequently electron) pulse, this is a very inefficient way to utilize the energy of the rf pulse. Thus in the conventional single-bunch scheme, only a single rf cycle out of thousands of cycles in an rf pulse is utilized for acceleration, even though the rf pulse length is usually much longer than the filling time of the accelerating cavities.
In principle, a train of many electron micro bunches can be accelerated synchronously in individual rf cycles within a single macro rf pulse, provided that the beam loading on the rf pulse is not severe. Such a multi-bunch acceleration scheme may require a very high rep rate laser. The repetition rate of a laser beam is inversely related to its peak power. Current technology limits the maximum rep rate of a laser with a milliwatt level of peak power to a few kilohertz, which is insufficient for the purpose of generating electron bunches typically a few nanoseconds apart for multibunch acceleration. Furthermore, even if the laser rep rate can be increased to a few hundred megahertz as required, only several thousands of these pulses would be needed during each macro rf pulse, and therefore all other laser pulses between macro rf pulses are unnecessarily wasted.
An example of a prior art approach is current work being carried out at the Rutherford Laboratory where multiple laser pulses are generated with a low-power, continuous wave (CW) laser and selected pulses are then amplified to high power using an array of pulsed amplifiers. For photoinjector application, the duration and rep rate of the amplified pulses are dictated by those of the high power rf source. Thus only the laser pulses that fall within the time structure of the pulsed amplifiers gain energy, and the rest of the CW laser pulses are discarded.
What is thus desired is to provide a multiple laser pulse train having a predetermined macro and micro time structure with an efficient device not using a CW laser.